Superconducting linear composite articles for high-magnetic field using Nb.sub.3 Sn and V.sub.3 Ga as linear superconducting material have been used heretofore. Also as a superconducting material for alternating current, Nb-Ti superconducting, multifilamentary wire is being put to practical use. See, for example, Superconductor Materials Science, Metallurqy, Fabrication and Application (NATO ADVANCED STUDY INSTITUTES SERIES, Series B: Physics) edited by Simon Foner and Brian B. Schwartz, Chapter 2, "Practical superconducting materials", pp. 63-67.
The composite articles containing Nb.sub.3 Sn and V.sub.3 Ga as linear superconducting material are manufactured by a process comprising drawing a composite composed of a copper alloy (Cu-Sn alloy or Cu-Ga alloy) and Nb or V, while effecting an intermediate annealing in vacuum at temperatures ranging 500-600.degree. C., repeating this processing several tens times and there after subjecting the product to a heat-treatment for diffusion reaction.
During the drawing, oxygen-free copper is compounded into the linear material for stabilization, but it necessitates insertion of tantalum foil, niobium foil, etc. into interspaces among the linear materials as diffusion barriar, in order to avoid diffusion reaction between the oxygen-free copper and the copper alloy.
Therefore, the production process of heretofore known Nb.sub.3 Sn- and V.sub.3 Ga-containing linear composite articles as above described involves very cumbersome fabrication steps and reduction in their production cost is difficult. Still more, upper critical magnetic field of whereby obtained Nb.sub.3 Sn- and V.sub.3 Ga containing superconducting material is only about 20T, and their ability to provide high-magnetic field is limited.
On the other hand, first practical ultra-fine multifilamentary composite wire showing extremely little alternating-current loss was provided by the use of Nb-Ti alloy. However, the critical temperature of this Nb-Ti alloy is 9K, and when it is used in liquid helium (4.2K), the temperature margin is only 4.8K. Linear composite articles of such a small temperature margin are not very advantageous for the use as superconducting magnet for alternating current, because a constant amount of heat generation is always occurred in the magnet during operation, and at temperatures close to, or exceeding, the critical temperature, the magnet performance is deteriorated or stopped.
On the other hand, Nb.sub.3 Al compound is known to have critical temperatures (Tc) ranging 15-19K, upper critical magnetic field of 30T, aforesaid temperature margin of at least 10 K and superior superconducting characteristics to either of the aforesaid Nb.sub.3 Sn-V.sub.3 Ga composite articles or the Nb-Ti alloy ultra-fine multifilamentary wire.
However, Nb.sub.3 Al compound had a serious drawback for its practical use, that it was very difficult to make it a linear material, particularly a ultra-fine gauge, homogeneous linear material. That is, it has been reported as a result of laboratory level experiments of compressing to solid a powdery mixture of Nb and Al, drawing the solid and heat-treating the same, that the resultant relatively short, linear composite article manifested excellent superconducting characteristics as above-described. However, it is extremely difficult to form long, filamentary product, particularly long, ultra-fine and homogeneous filamentary product, from this Nb.sub.3 Al compound, and for this reason it has not yet been commercialized.
The foregoing facts are taught in the following references:
(i) Advances in Cryogenic Engineering, Vol. 34, "Materials" Pages 461-468. PA0 (ii) IEEE Transactions on Magnetics, Vol. Mag-21, No. 2, pages 756-759 (1985). PA0 (iii) IEEE Transactions on Magnetics, Vol. Mag-23, No. 2, pages 653-656 (1987).
One of the causes why the linear body of the Nb.sub.3 Al compound test-manufactured by the powder process on laboratory level as above has not been commercialized in spite of its excellent superconducting property lies in the extreme difficulties of preventing oxidation of Nb and Al powders serving as the starting material and of controlling their particle sizes.
On the other hand, manufacture of linear bodies of Nb.sub.3 Al compound through the steps of forming filaments or thin films of Nb and pure Al, compounding them, cold-drawing the composite material and heat-treating the same was also attempted. Because of the lower hardness of pure Al than that of Nb, however, Al shows abnormal deformation during the drawing, causing sometimes breakage of the linear body. Consequently fine and homegeneous filamentary product cannot be obtained, and hence the process has not been commercialized.